1. Field of the Invention
The present invention relates to a technique of converting tone image data into dot data expressed by the dot on-off state. More specifically the invention pertains to a technique of collectively processing a preset number of adjacent pixels as a unit so as to allow for quick conversion of image data, while keeping the picture quality of a resulting image at a sufficiently high level.
2. Description of the Related Art
Image display apparatuses that create dots to express an image on a display medium, such as a printing medium or a liquid crystal screen, are widely used as the output device of diverse imaging equipment. The image display apparatus is capable of locally expressing only two states, the dot on state and the dot off state. An image of continuously varying tones is expressible by adequately regulating the dot creation density according to the varying tone of the image.
In order to create dots at adequate densities according to the tone values of an image, the error diffusion technique is generally applied for determination of the dot on-off state in each pixel in the image display apparatus. The error diffusion method diffuses an error of tone expression arising due to creation of a dot or no creation of any dot in a target pixel to peripheral unprocessed pixels in the vicinity of the target pixel, and determines the dot on-off state in each of the unprocessed pixels such as to cancel the tone errors diffused from peripheral processed pixels. Application of the error diffusion method enables the dot on-off state to be determined in each target pixel such as to cancel the error of tone expression arising in peripheral processed pixels, thus attaining creation of dots at adequate densities corresponding to the tone values of the image.
The error diffusion method advantageously allows for dot creation at adequate densities according to the tone values of the image, thus ensuring a high-quality resulting image. This method, however, requires diffusion of the tone error to peripheral unprocessed pixels every time the dot on-off state in each target pixel is determined. In the case of an image consisting of a large number of pixels, such diffusion undesirably extends the total image processing time. In order to solve such drawbacks, the inventors of the present invention have developed and filed a technique of joining a preset number of adjacent pixels into a unit and determining the dot on-off state with diffusion of the tone error by units (JAPANESE PATENT APPLICATION No. 2001-238117).
In the method of determining the dot on-off state by units, an identical series of processing is repeated for the respective units. This may result in creation of a dot at identical pixel positions in multiple units. These dots may be visually recognized as a periodical pattern and worsen the picture quality of the resulting image. The potential appearance of a periodical pattern in the process of determining the dot on-off state by units is not restrictive to the error diffusion method but may be found in other known methods.